How can I monitor daily spending on AWS?

Our infrastructure is on AWS. I want to get a daily report on how much spent on the previous day. What is the best way to do it?

Update

AWS has just announced the general availability of the functionality to Monitor Estimated Charges Using Billing Alerts via Amazon CloudWatch (it apparently has been available to AWS premium accounts already since end of 2011, see Daniel Lopez' answer to Is there a way to set Amazon AWS billing limit?):

We regularly estimate the total monthly charge for each AWS service that you use. When you enable monitoring for your account, we begin storing the estimates as CloudWatch metrics, where they'll remain available for the usual 14 day period. [...]

As outlined in the introductory blog post, You can start by using the billing alerts to let you know when your AWS bill will be higher than expected, see Monitor Your Estimated Charges Using Amazon CloudWatch for more details regarding this functionality.

This is already pretty useful for many basic needs, however, using the CloudWatch APIs to retrieve the stored metrics yourself (see GetMetricStatistics) actually allows you to drive arbitrary workflows and business logic based upon this data, and of course you could generate a daily report on how much spent on the previous day like so as well.

Regarding the latter, the scope of this offering is stressed as well though:

It is important to note that these are estimates, not predictions. The estimate approximates the cost of your AWS usage to date within the current billing cycle and will increase as you continue to consume resources. [...] It does not take trends or potential changes in your AWS usage pattern into account. [emphasis mine]

That is, the granularity of the reported metrics has yet to be analyzed (I see data points every 4 to 8 hours, but not necessarily updated values every time, as one would expect actually), so deriving a sufficiently precise daily report might require some statistical post processing.

Initial Answer

Unfortunately this is less straight forward than one would think, especially given that the desired data can be inspected manually via your account. There are two monitoring options one would expect:

• notifications via email/RSS/etc.
• API access to the data

Neither AWS nor any other IaaS/PaaS/SaaS vendor I'm aware of does offer API access to their accounting data currently (maybe due to the potential financial/legal implications), making any form of 3rd party integration (which would be easy to do nowadays) cumbersome at best, insofar you need to resort to web scraping to retrieve the data in the first place.

Fortunately a new offering from Cloudability [link removed after discontinuation of free tier] has entered the stage recently to do just this for you in a professional and vendor agnostic way, we are using it with great success already for AWS specifically - you'll currently receive a daily (or less frequent) report of your monthly spending only though, i.e. not broken down to your daily spending yet. Adding the daily increase would be trivial of course, so I'd hope and expect they'll make more information like this available over time.

Their approach to pricing [link removed after discontinuation of free tier] is refreshing as well (despite being obvious) and simply tied to your own cloud spending, thus should pay for itself as soon as you realize respective saving potential (they don't charge anything at all if you spend less than $2.5k/mo).
Update 20121016: Unfortunately Cloudability has changed their pricing model to a more common one, which still includes a free tier (and is reasonable priced in general), but removes access to advanced features therein, which I considered a refreshingly fair and smart approach for users with small budgets, who might still be multipliers elsewhere or upgrade once growing into it.
Update 20150115: Unfortunately Cloudability has chosen the path of many freemium SaaS vendors and finally discontinued the free tier entirely: From February 1, we will no longer offer the Cloudability Free edition that you are using today.

Update 20120427

The former caveat (kept for reference below) of requiring your main AWS credentials doesn't apply anymore - AWS recently introduced New IAM Features: Password Management and Access to Account Activity and Usage Reports Pages:

This new feature allows you to create separate and distinct IAM users for business and technical purposes. You can grant your business users access to the Account Activity and/or Usage Reports pages of the AWS website to allow them to access billing and usage data without giving them access to other AWS resources such as EC2 instances or files in S3

Cloudability has now integrated this as well, thus you don't need to hand them your main AWS credentials anymore or spent the extra effort to establish Consolidated Billing just to gain insight into your cloud spending, see How to Setup Amazon IAM (Identity Account Management) for details.

Former Caveat

There is one caveat one should be aware of upfront though:

In order to access your data you'll need to hand them your main AWS credentials, because otherwise they can't scrape your account, obviously. For AWS in particular you can still avoid this by facilitating Consolidated Billing, where you consolidate payment for multiple Amazon AWS accounts [...] by designating a single paying account, which in turn has no access to your computing resources and data.

Using awscli tools, you can get your month-to-date total:

$ aws --region us-east-1 cloudwatch get-metric-statistics \
    --namespace "AWS/Billing" \
    --metric-name "EstimatedCharges" \
    --dimension "Name=Currency,Value=USD" \
    --start-time $(date +"%Y-%m-%dT%H:%M:00" --date="-12 hours") \
    --end-time $(date +"%Y-%m-%dT%H:%M:00") \
    --statistic Maximum \
    --period 60 \
    --output text | sort -r -k 3 | head -n 1 | cut -f 2

2494.47

Totals from two different days can be subtracted to get the daily delta. Or, an estimate can be obtained in one go by increasing the time window (end-time - start-time) to 24h and subtracting the earliest data point from the latest.

Notes:

• assumes you have enabled monitoring of your estimated charges
• assumes you have GNU date (for other dates, e.g., on OSX, date -v-12H may work)
• your Currency may vary
• Amazon updates the CloudWatch metric every ~4h, so querying for a time window smaller than 4h may not return any data points. I've used a 12h window to be extra safe.
• I wanted to include a link to the blog post on which my answer was based, but apparently I need 10 reputation points to post more than 2 links. :)